1. Field of the Invention
The present invention relates to an accelerated thermal process under acidic cooking for the production of precooked corn flour and, more particularly, it relates to a continuous partial removal of the corn hull fraction with an acidic reducing agent applied to the production of instant corn flour for the manufacture of arepa and tortilla and the like.
2. Description of Related Art
Tortilla and corn dough (masa) are traditionally made from whole corn kernel which is cooked and/or stepped in calcium hydroxide (lime) or sometimes in sodium hydroxide (lye) or wood ashes, in order to produce a reaction known as "nixtamalization". The nixtamalized corn is further steeped, washed with partial hull removal and finally stone ground into a corn dough. Corn dough texture is determined by factors such as corn variety or cultivar, endosperm hardness as well as the water uptake (dough yield) and degree of starch gelatinization mainly during corn "cooking" (thermal) and cooked corn (nixtamal) "grinding" (mechanical) operations.
During alkali-cooking and/or steeping, there are chemical and physical changes such as nutrient losses along with partial hull removal and gelatinization in the corn kernel. The most important nutritional modifications are: an increase in the calcium level with improvements in the Ca to P ratio, a decrease in insoluble dietary fiber and zein-protein, a significant reduction in thiamin and riboflavin, an improvement of the leucine to isoleucine ratio reducing the requirement for niacin and reduction of the aflatoxins by leaching into the lime-water.
The cooking method, raw corn quality and cooker-type (batch or continuous) used have been proposed as the critical variables which determine solid losses in lime-water residue (nejayote). This wastewater by-product was composed of corn solids ranging from 1.5% (batch-cooker) to 2.5% (continuous-cooker). Dry solid matter includes an average of 50-60% dietary fiber, 15-20% ash, 15% starch, 5-10 % protein and 5% fat. The corn pericarp makes up 4-6% of the kernel dry weight (Watson, 1987), It also contains 67% hemicellulose, 23% cellulose and 0.6% soluble-fiber. It is estimated that dietary fiber in the pericarp (4.9%) and endosperm (2.6%) make up 80% of the total dietary fiber (9.5% in a dry-weight basis).
During the formation of corn dough, grinding disrupts the swollen and partially gelatinized starch granules and further distributes the hydrated soluble starch, protein and fiber fractions around the ungelatinized fraction of the endosperm-germ granules.
Tortilla is the main edible corn product in Latin America. It is a flat, round, unleavened and baked thin pancake (flat-cornbread) made from fresh masa or corn dough prepared from industrial nixtamalized corn flour (masa flour). It might be mentioned that tortilla, when manually or mechanically elaborated and without additives of any kind, has a maximum shelf life of 12 to 15 hours at room temperature. Afterwards they are spoiled by microorganisms and become hard or stale (retrogradation) due to a physicochemical change in the starch constituent of either stored or reheated tortilla It is known that tortilla when kept under conditions in which no moisture is lost (plastic package storage), nevertheless become inflexible with time and break or crumble easily when bent.
Properly processed industrial masa flour simplifies the production of tortilla products, because the customer eliminates management techniques required for wastewater treatment, securing, handling and processing corn into masa for tortillas and snacks. However, an instant corn flour potentially has the following quality cost disadvantages: high cost, lack of flavor, and poor texture in tortilla products or snacks prepared from masa flour.
There is nevertheless an increasing demand for and use of industrially prepared corn flours for the benefits they provide to the consumer, such as convenience, availability, incorporation of micronutrients and additives, low price and a more stable product quality (Sustain, 1997). The basic difference between tortilla (from Mexico and some Central American countries) and arepa flour (Venezuela and Colombia) is that for the former, whole corn is alkali-cooked and the hull is partially separated to obtain a lime-precooked corn flour (masa flour). On the other hand, arepa flour (precooked flour) is prepared from dehulled and degerminated corn without using an alkali-cooking treatment (INCAP, 1961). However, arepa flour is further enriched, by Venezuelan law, with B-vitamins and iron to improve the nutritional value of this traditional South American food mainly based on this baked thick pancake (flat-cornbread: Cuevas et al. 1985).
Corn processes can generate added value from their industrial operations in one of three approaches: developing new products from new hybrids, increasing the yield of traditional products from corn, and improving process efficiency at a lower unit cost. In the past, this has been done by methods and apparatus where the grain is cooked and/or steeped in a lime-water solution such as those disclosed in U.S. Pat. Nos. 2,584,893; 3,194,664; 4,326,455 and 4,513,018. These prior art methods for the industrial production of corn dough or masa flour involve accelerated cooking and steeping times with large amounts of solids losses (1.5-2.5%) in the liquid waste. In addition, essential nutrients such as vitamins and some amino acids are lost, depending on the severity of the cooking and washing operations.
Many and varied methods for the production of instant corn flour for tortilla involving reduced amounts of water with short process times and low energy requirements for a high yield of the end product have been developed, as reflected by the following U.S. Pat. Nos. 4,250,802; 4,329,371; 4,594,260 and 5,532,013. Although the above described prior art methods are capable of producing corn flour with increasing yields and processing efficiencies, a continuous process was still unavailable in the expanding masa flour industry for tortilla and the like.